Hydraulic drive for inertia loads



y 1951 R. TYLER HYDRAULIC DRIVE FOR INERTIA poms Filed April 28, 1948 ATTORNEY mm mm mm B 6 5 R V P 0 m m m Y mm o E T m ww W M mm mm mm Rum mm mw v I 0 8 um in \2 N 0 5 \\\\\\\\\R A Y & .\o R Q row B m m o l 5 mi 1mm N w m 2 2 cu m 9 mm 5 mm v 1. 2 mm mN 1 1 mm m um n t NV a c on N NN Ll IL 5 V3] 2 .n 3 W L mv w n m w i, 1|- fil Patented May 15, 1951 HYDRAULIC DRIVE FOR INERTIA LOADS Ransom Tyler, Greenfield, Wis., assignor to The Oilgear Company, Milwaukee, Wis., a corporation of Wisconsin Application April 28, 1948, Serial N 0. 23,747

7 Claims. (01. 60-52) This invention relates to hydraulic drives for inertia loads. The drive to which the invention relates in particular includes a hydraulic motor for driving a load, a pump for supplying motive liquid to the motor to energize it, two fluid channels connecting the pump to the motor and forming therewith a hydraulic circuit, and flow control means for reversing or for starting and stopping the flow of liquid in the circuit to thereby cause the motor to move its load in opposite directions alternately or to start and stop its load intermittently.

For the purpose of illustration, the invention will be explained as being embodied in a drive for reciprocating a saw mill carriage which is adapted to carry a heavy timber past a saw to enable the saw to take cuts from the timber. It is to be understood however, that the invention is is not limited to a drive for such use or to a drive for reciprocating a load and that the invention may be embodied in drives for other purposes and in drives which will start and stop loads intermittently.

The flow control means of a hydraulic drive for a saw mill carriage ordinarily is shifted to effect reversal of the carriage when the motive liquid is flowing at a maximum rate from the pump through the first of the two channels to the motor and causing it to drive the carriage at a maximum speed and to discharge liquid at a maximum rate through the second of the two channels to the pump.

The flow control means may be shifted very quickly to divert the motive liquid from the first channel into the second channel but shifting the flow control means does not immediately effect reversal of the carriage for the reason that the inertia of the carriage and the parts movable in unison therewith is so great that a large force and a considerable interval of time are required to decelerate the carriage from maximum speed to zero speed and a corresponding force and interval of time are required to accelerate the carriage from Zero speed to maximum speed. Therefore, the carriage will continue to move in the same direction after the flow control means is operated and it will cause the motor to continue to operate in the sam direction and to continue to discharge liquid into the second channel until the carriage is decelerated to zero speed. Then the liquid discharged by the pump into the second channel will cause the motor to start the carriage moving in the opposite direction and to gradually accelerate it to maximum speed.

In the prior drives of this kind, the new control means diverted themotive liquid very quickly from one channel into the channel into which the motor was discharging so that the full volume of the pump and all of the liquid discharged by the motor had to be exhausted through the pump relief valve until the motor was decelerated to zero speed and thereafter liquid was discharged through the relief valve at a rate which gradually decreased from nearly the full pump delivery at zero motor speed to zero at maximum motor speed.

Exhausting liquid through a relief valve causes the oil to become heated and gassified, thereby decreasing the usefulness of the liquid and wasting power. Since the pump relief valve necessarily is set to open at a pressure higher than the normal working pressure, the prior drives couldoperate only a short time before the liquid became excessively hot and sometimes the liquid became sohot that the drive could not be successfully operated.

The present invention has as an object to provide a drive for inertia loads which may be operated continuously without the motive liquid becoming excessively heated.

Another object is to provide a hydraulic drive capable of reversing or starting and stopping an inertia load without causing any substantial part of the energy stored in the motive liquid to be transformed into heat.

Another object is to provide a hydraulic drive which includes a motor and a variable displacement pump and means for so regulating the displacement of the pump that little if any liquid is exhausted through a relief valve when the motor is started, stopped or reversed.

Other objects and advantages will appear from the following description of the hydraulic drive shown schematically in the accompanying drawing in which the views are as follows:

Fig. 1 is a diagrammatic view of a hydraulic drive in which the invention is embodied.

Fig. 2 is a view of a control linkage, the view being taken on the irregular line 22 of Fig. 1 but showing the parts in positions different from those shown in Fig. 1.

For the purpose of illustration, a drive embodying th invention has been shown employed to reciprocate the carriage I of a saw mill. Since the saw mill forms no part of the invention, only parts of the carriage have been shown.

Carriage l is adapted to be reciprocated by a hydraulic motor 2 which may be of the reciproeating type but which has been shown as being of the rotary type and as being mechanically 3 connected to carriage l by suitable gearing 3 so that, when motor 2 rotates in one direction or the other, it will drive carriage I in one direction or the other.

Liquid for energizing motor 2 is supplied thereto by a pump 4 which is continuously driven when the'drive is in operation and which is connected to motor 2 by two channels 5 and 6 which form therewith a closed hydraulic circuit. The pressure created by pump 4 in either direction of pump delivery is limited by one or the other of two high pressure relief valves 1 and 8 which have been shown connected between'channels 5 and 6. In practice, however, valves 7 and 8 are arranged within the casing of pump 4 and discharge into a base (not shown) which supports pump 4 and constitutes a reservoir from which pump 4 is supplied with liquid.

Pump 4 may be of any suitable type but it has been indicat'ed as being of the general type shown in Patent No. 2,227,814. Since such pumps are well known and in extensive use, it is deemed suiiicient to state herein that the pump has its pumping mechanism arranged within a displacement varying member or slide block 9 which is arranged within the casing H!- of the pump, that pump displacement will be zero when slide block 9 is in its central or neutral position, and that the pump will deliver liquid in a direction and at a rate dependent upon the direction and distance slide block 9 is shifted from its neutral position.

Slide block 9 is adapted to be moved upward in respect to the drawing by a piston H which engages or is fixed to one side of slide block 9 and is fitted in a cylinder 1-2 which is formed in the sidewall of casing l9 and is closed at its outer end by a cap 13. Cylinder I2 is supplied with operating liquid in which pressure is continuously maintained as will presently be explained.

Slide block 9 is adapted to be moved downward in respect to the drawing'by a piston l4 which is larger than piston II and engages or is fixed to the opposite side of slide block 9. Piston I4 is fitted in a cylinder l5-which isformed in the side wall of easing l9 and is closed at its outer end by a head l6 having'a bore ll formed therein in axial alinement with a bore l8 which extends through piston I4 and through an extension or hub l9 formed upon the outer face of piston l4. The inner end of bore iiicommunicatesat all times with the interior of casing 10 as by means of a slot 29 formed in slide block 9.

Liquid is adapted to flow into and out of cylinder l5 through a port 21 which extends through the wall of hub 19 into communication with bore [8. Port 21- is controlled by a rotary valve 22 which is closely fitted in bores I? and I8 and has two spiral grooves 23 and 24 formed in its periphery at opposite sides of port 2! and spaced apart a distance equal to the diameter of port 2!.

One end of groove 23 extends to the end of valve 22 so that groove 23 is at all times in-communication with the inner end of bore! 8. Groove 24 communicates with one end of an internal passage 25 which extends through valve 22 and has its other end in communication with an annular groove 26 which is formed in the wall of bore i1.

Liquid for energizing servo-motors I2 and l4l5 is supplied by a gear pump 39 which for the purpose of illustration has been shown outside of casing 10 but which in practice is arranged within casing l9 and is driven in unison with pump 4.

Gear pump 30 draws liquid through a channel 3| from the base or reservoir of pump 4 and discharges it into a supply channel 32 which has one end thereof connected to end head I6 in communication with groove 26 and the other end thereof connected through a pressure responsive Valve 33, to be presently described, and a channel 34 to cylinder i2. Gear pump 99 discharges liquid at a rate in excess of requirements and the excess liquid is exhausted through a low pressure relief valve 35 and a channel 36 into the base of pump 4 so that pump 39 maintains in channel 32 a pressure equal to the resistance of relief valve 35.

Ifhe arrangement is such that, when valve 22 is rotated in one direction, groove 24 will open to port'2l and then gear pump liquid will flow from channel 32 through groove 26, passage 25, groove 24 and port 25 into cylinder 25. Since piston I4 is larger than piston H, the liquid entering cylinder IE will cause piston 14 to move slide block 9 and piston l I downward in respect to the drawing and piston II will eject liquid from cylinder i2 through channel 54 and valve (itinto channel 32. Piston [4. will continue to move slide block 9 downward until port 25 moves out of communication with groove 24. or until slide block 9 reaches the limit of its movement. Moving slide block 9 downward below its neutral position causes pump 4 to discharge liquid into channelfi.

Whenvalve 22 is rotated in the opposite direction, groove 23 will open to port 2| so that liquid can escape from cylinder 15. Then-thegear pump liquid continuously supplied to cylinder l2 will cause piston II to move slide block 9 and piston l4 upward in. respect to thedrawingand piston M will eject liquid from cylinder l5 throughportv 2|, groove 23, bore I8 and; slot 29 into casing it from whence the oil drains into the base of. pump 4. Piston II will continue tomove slide block- 9 upward until port 2! moves out of communication with groove 23 or until slide block 9. reachesthe limit of its movement. Moving slide. block 9 upward above its neutral position causes. pump.4 to discharge liquid into channel 5.

Valve 22 may be rotated either manually or in; response to carriage l reaching a given position. during its movement in'each direction. As shown,. valve 22 isadapted to be rotated by a lever 49 which is fixed to the outer end thereof and which. also restrains valve 22 fromaxial movement. The. freeend of lever 45 has a slidable pivotal connection with one arm of a bell crank lever 41 which; is pivoted upon a. stationary shaft 42; The other arm oflever 4] is connected by an adjustable link. 43 to a lever 44 which is fixedupon a shaft 45 mounted in suitable bearings one of which is indicated at46.

Shaft 45 is adapted to be rotated in one direction or the other either by a hand lever 4! fixed thereon or by oneor the other of two reversing ears 48: and 49 which are fixed upon shaft 45 and adapted to be operated, respectively, by two dogs 50 and 5| which are fixed to carriage i in adjusted positions.

Valve '22- or'dinarily is rotated almost instantly from one position to another regardless of Whether it is rotated manually or is rotated in response to carriage l reaching a given position. Ifno means were provided for limiting its rate of movement, slide block 9 would move almost as fast-as valve 22 was rotated and would vary the rate of delivery of liquid from pump 4 so rapidly that large volumes of liquid would be-exhausted through r'eliefvalves I and 8 as explained above.

In a drive embodyingthe present. invention, slide block 9 can move freely until the pressure in one or the other of channels 5 and 6 is nearly high enough to open the relief valves 1 or 8 con,- nected to the channel 5 or 6 containing the high pressure. Then the pressure in that channel will cause valve 33 to throttle the flow of liquid into or out of cylinder l2 and thereby limit the rate at which slide block 9 can move.

As shown, valve 33 includes a valve body 55 having an axial bore 56 in the central portion thereof, two annular grooves or ports 51 and 5! formed in the wall of bore 56 and two counterbores or pressure chambers 58 and 58 arranged at opposite ends of bore 56 concentric therewith. Ports 5'! and 5'! have channels 32 and 34 connected thereto respectively. Pressure chambers 58 and 58 are connected to channels 5"and 6, respectively, by two channels 59 and 60.

Communication between ports 51 and 51 is adapted to be restricted or interrupted by a piston valve 6| which is slidably fitted in bore 56. The right hand end of valve 6| is engaged by a spring retainer 62 which normally is held against the annular end wall of chamber 58 by a spring 63 and which has a passage 64 extending therethrough so that liquid can flow freely between chamber 58 and the adjacent end portion of bore 56. The tension of spring 63 may be adjusted by means of shims inserted between its outer end and the outer end wall of chamber 58 but it has been shown as engaging a sprin retainer 65 which is engaged by an adjusting screw 66 threaded through the end wall of chamber 58.

Since the two halves of valve 33 are identical, further description is deemed unnecessary as like parts have been indicated by the same reference numerals with the exponent a added to the reference numerals applied to the parts in the left hand half of valve 33.

Operation With the parts in the positions shown in Fig. 1 and with pumps 4 and 30 running, pump 4 is at zero stroke so that no liquid is being discharged thereb and pump 30 is discharging through channel 32, relief valve 35 and channel 36 into the base of pump 4.

Assuming that carriage I has a heavy load arranged thereon and that it is to be reciprocatecl for a period of time after being started moving toward the left in respect to the drawing, the drive will operate as follows:

The drive may be started by swinging hand lever 4'! upward, as indicated in Fig. 2, which will cause valve 22 to be rotated in a direction to open spiral groove 24 to port 2|. Then gear pump liquid will flow from channel 32 through groove 26, passage 25, groove 24 and port 2| into cylinder l5 and cause piston l4 to move slide block 9 downward, thereby causing pump 4 to deliver liquid through channel 6 to motor 2 to energize it.

Slide block 9 will start to move and cause pump 4 to start to deliver liquid substantially as soon as valve 22 starts to rotate. The rate at which slide block 9 moves and the rate at which pump 4 discharges liquid will increase at substantially the rate at which valve 22 is rotated but, due to the inertia of carriage and the parts movable therewith, motor 2 cannot start until pump 4 has created a high pressure and it cannot accelerate as fast as the delivery of pump 4 increases.

Pump 4 thus tends to deliver liquid faster than the liquid can enter motor 2 which causes pressure to rise in channel 6 and, when the pressure becomes high enough, motor 2 will start but pump 4 continues to tend to deliver liquid faster than the liquid can enter motor 2 so that the pressure" continues to rise until it reaches a predetermined maximum which is determined by'the resistance of spring 63 and which is a little lower than the" pressure required to open relief valve 8. Then liq-' uid will flow from channel 6 through channel 6|), pressure chamber 58 and passage 64 into bore 56 and move valve 6| toward the right against the resistance of spring 63.

Moving valve 6| toward the right restricts port 51 and reduces the rate at which liquid can escape from cylinder l2, thereby restricting the movement of slide block 9 and reducing the rate at which pump delivery increases. If lever 41 is operated very suddenly as is customary, pump delivery may increase so rapidl and pressure may rise so suddenly and so high that valve 6| would be moved far enough to completely close port 51- and thereby stop further movement of slide block 9 and further increase in pump delivery until motor 2 is started and then the pressure would drop slightly and permit spring 63 to move valve 6| far enough toward the left to partly open port El 50 that liquid could escape from cylinder l2 at a limited rate and permit piston I5 to move slide block 9 downward to increase pump delivery at the rate required to maintain the maximum pressure in channel 6.

Since the force required to start and accelerate a load is greater than the force required to keep the load moving after it has been accelerated to a. given speed, slide block 9 will move downward and increase the delivery of liquid from pump 4 at the rate required to maintain the maximum pressure in channel 6 until motor 2 has been accelerated to its running speed and then the pressure will drop and permit spring 63 to move valve 6| to its central position.

The pressure created by the pump is thus maintained at a predetermined maximum so that the time required to accelerate carriage to running speed is reduced to a minimum without causing a large volume of liquid to be exhausted through a relief valve as would be the case if movement of displacement varying member were not controlled in response to pump pressure reaching a predetermined maximum.

Motor 2 will drive carriage toward the left at high speed until shaft 45 is rotated counterclockwise in respect to Fig. 2 either by hand lever 41 being moved downward manually or by dog 56 engaging reversing ear 48 and swinging it toward the left.

Rotating shaft 45 counterclockwise causes valve 22 to be rotated in a direction to open.

spiral groove 23 to port 2| so that liquid can. escape from cylinder l5 and permit the gearpump liquid supplied to cylinder l2 to cause pis thereby start reversing pump 4.

Slide block 9 will start to move substantiall as: soon as valve 22 starts to rotate and it will very quickly reduce the rate of pump delivery below" therate required to keep motor 2 operating at; its current speed but, due to the inertia of car-; riage and the parts movable therewith, motor 2 will continue to operate in the same directionand continue to discharge liquid through channel 5 to pump 4.

Slide block 9 will tend to reduce pump displacement to such an extent that all of the liquid discharged by motor 2 cannot enter pump 4, thereby causing motor 2 to create pressure in channel. 5. Since the energy which enables motor 2 to create pressure in channel 5 is derived from care seeps-e i Ij ,ifibfb Q I a? erase uteri carriage I ztogquickly decelerate to. Zero speed.

. The: pressure created :by moto 2 w l v t i -until it reaches a, predetermined max mum which determinedgb y; the res stance er spring 83 1 and which is a 1 'littielower '59, pressure chamber 58 and passage't l into bore '58 and move valve iil toward the left against the resistance of spring 53 Moving valvetl toward the left restricts port 51 and reduces the rate at which liquid can enter cylinder I2,thereby restricting the upward move- 'ment of slide block 9 and reducing the rate at which pump delivery is reduced. Since valve BI -is operated 'by the pressure in channel 5, it will limit'the fiow of liquid into cylinder I2 to the rate necessary to enable piston I i to reduce pump dlS- placement just fast'enough to cause motor 2 to maintain the predetermined maximum. pressure in channel 5 until motor 2 is decelerated substantially to zero speed.

When motor 2 reaches zero speed, piston II will have moved slide block 9 far enough to cause pump 3 to start discharging liquid into channel 5. Sincethe movement of slide blocleil is controlled by the pressure in channel 5, pump i-will maintain the same maximum pressure in channel 5 so-that the instantthat carriage I stops moving toward the left motor 2 is exerting suificie'nt torque to start its moving toward the right.

Motor 2 will gradually accelerate carriage I and piston i! will continue to move slide block 9' just fast-enough to enable pump t to maintain the predeterminedmaximumpressure in channel 5 until motor 2 approaches its running speed and then, since the force required to keep carriage 4 I moving at a given speed is less than the force required to accelerate it to that speed, the pressure in channel 5 will drop and permit spring. 63

to return valve 6| to its central position.

Motor 2 will drive carriage I toward the right at high speed until shaft &5 is rotated clockwise in respect to Fig. 2 either by hand lever 61 being moved upward manuall or bydog fiI e gaging reversing ear 33 and swinging it toward the right. Rotating shaft Q5 clockwise causes valve 22 to be rotated in a direction to open spiral grooves 24 to port ZI so that liquid can enter cylinder i5 and cause piston It to start moving slide block 9 downward. Then the drive will be reversed in the manner described above except that slide block 9 will be moved downward, motor 2 will discharge liquid into channel t while being decelerated, pump lwill discharge liquid-into channel 6 after motor 2 has been decelerated to zero speed, and the pressure in channel 6 will cause valve 6! to throttle the flow of liquid out of cylinder I2 as slide bloclc 9 moves downward.

Motor 2 will continue to drive carriage I in opposite directions alternately until valve- 22 is rotated to its neutral position as shown in Fig. 1 which will cause slide block d to return to its neutral position. Slide block 9' will start to move as soon as valve 22 starts to rotate but carriage I will continue to move and to drive motor 2 "movement of slide block 9 to the raterequired to enable motor 2"to create pressure until it Stops.

' Whemthe-pump' or motoris creating' pressurein I o'iie' side of the. circuit, there is flinsufiicient liduidin the-other side of the circuit t supply the ,pump or motor which is creating the' pres .surebut this deficiency is made up by liquid supplie'dlto! the lowpressure side of the circuit either directlyfromthe reservoir or by gear pump 30 through one or the other of .two check valves one of which is connected toeither side ofthe circuit and which permits liquid to "flow intoflthe circuit but prevents 'fiow'in the opposite directron. Since such an arrangement is well'known andfin extensive use, it has' been omitted from thedrawing in order to avoid complicating the View. r V v v "If the displacement varying memb'ervof the pump were permittedtdmove.freely in lajdrive for inertia loads as .is customary in'the .prior drives of this character, the pump would-exhaust liquid throughla relief-valve while theload was being started and'being acceleratedto r-unning speed, the motor would exhaust liquid vthrough .arelif valve whilethe load was beingjstfip p'd,

and du'ring reversal both the pump and the motor would exhaust through a relief valve while the load was beingxdecelerated. to zero speed and thn; the, pump would exhaust liquid. through a relief valve'wh'ile the loadwasbeing accelerated torunning.- speed. A drive embodying the p'resentinvention is capable of starting, stopping and reversing an inertia loadjust as quickly .asthe prior drivesv but substantiallyc no' liquid is exhausted-through the relief valves.

The invention herein. set forth j is. s sc'epume of veiriousmodificttions and. adaptations without departing fromtlienscope thereof which is hereby claimed as follows 7 l. 'A hydraulic drive comprising a hydraulic motor for driving an inertia load, a pump for supplying motive liquid to said motor to energize the samgandhaving a displacement varying member movable toand from a neutral position :to vary the rate of pump -:delivery,r flui d. channels connecting said pump-t0 -sai dmotor; and forming therewith a hydraulic circuit; hydraulic servo-motor means for moving saiddisplacement varying member, 1 means supplying opera-ting liduid to c-s aid servo-motor means as-first valve l for controlling theoperation of said servo-motor means,and a second valve operablein response to the pressure in said circuit-reaching apredetermined maximum forlimiting the rate at which said 'ser-vomoto-r means can-move said .7 displacement varying member.

,;2. A hydraulic drive comprising ahydraulic ,motorf for driving an inertia load aspumpl for supplying motive liquid to said motor to energize the-same and havinga displacement. varying. -member movable toand from a neutral 'position to vary the rate. of pump-delivery, fluid channels connecting said pump to said motor and qforming: therewith a hydraulic circuit;.hydraulic servo-motor means for moving said displacement varying member, means supplying operating liquidtox'said servo-motor means,- a first va'lve for -controlling, theyo-peration of said servo-motor limitthe'ra'tezof movement of said displacement 1 varying mem er.

3. A" hydraulic drive =comprising= a 'hydra'ulic mbtor for driving an inertia load, a -pump for -supplying -moti-ve lic'guid to said motor to ener- 76 g'il't thesarn'e and having a displacementvarying member movable in opposite directions from a neutral position to vary the rate and direction of pump delivery, fluid channels connecting said pump to said motor and forming therewith a hydraulic circuit, servo-motor means for movin said displacement varying member, and means responsive to the pressure in either side of said circuit reaching a predetermined maximum for limiting the rate of movement of said member.

4. A hydraulic drive comprising a hydraulic motor for driving an inertia load, a pump for supplying motive liquid to said motor to energize the same and having a displacement varying member movable in opposite directions from a neutral position to vary the rate and direction of pump delivery, fluid channels connecting said pump to said motor and forming therewith a hydraulic circuit, hydraulic servo-motor means for moving said displacement varying member, means for supplying operating liquid to said servo-motor means, a first valve for controlling the operation of said servo-motor means, and a second valve operable in response to the pressure in said circuit reaching a predetermined maximum for limiting the rate at which said servo-motor means can move said displacement varying member.

5. A hydraulic drive comprising a hydraulic motor for driving inertia load, a pump for supplying motive liquid to said motor to energize the same and having a displacement varying member movable to and from a neutral position to vary the rate of pump delivery, fiuid channels connecting said pump to said motor and forming therewith a hydraulic circuit, a large servomotor for moving said member in one direction, a small servo-motor for moving said member in the opposite direction, a source of operating liquid, means including a valve for connecting said large servo-motor to said source and to exhaust alternatively, and means for connecting said small servo-motor to said source including a valve operable in response to the pressure in said circuit reaching a predetermined maximum for throttling the flow of liquid into and out of said small servo-motor.

6. A hydraulic drive comprising a hydraulic motor for driving an inertia load, a pump for supplying motive liquid to said motor to energize the same and having a displacement varying member movable to and from a neutral position to vary the rate of pump delivery, fluid channels connecting said pump to said motor and forming therewith a hydraulic circuit, a large servo-motor for moving said member in one direction, a small servo-*notor for moving said member in the opposite direction, a source of operating liquid, means including a valve for connecting said large servo-motor to said source and to exhaust alternatively, and means for connecting said small servo-motor to said source including a valve operable in response to the pressure in either side of said circuit reaching a predetermined maximum for throttling the flow of liquid into and out of said small servo-motor.

7. A hydraulic drive comprising a hydraulic motor for driving an inertia load, a pump for supplying motive liquid to said motor to energize the same and having a displacement varying member movable in opposite directions from a neutral position to vary the rate and direction of pump delivery, fluid channels connecting said pump to said motor and forming therewith a hydraulic circuit, a large servo-motor for moving said member in one direction, a small servomotor for moving said member in the opposite direction, a source of operating liquid, means including a valve for connecting said large servomotor to said source and to exhaust alternatively,

and means for connecting said small servo-motor to said source including a valve operable in response to the pressure in either side of said circuit reaching a predetermined maximum for throttling the flow of liquid into and out of said small servo-motor.

RANSOM TYLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,974,138 Ferris et a1 Sept. 18, 1934 2,056,896 Douglas Oct. 6, 1936 2,114,005 Tyler Apr. 12, 1938 2,238,060 Kendrick Apr. 15, 1941 2,238,061 Kendrick Apr. 15, 1941 2,238,063 Kendrick Apr. 15, 1941 2,389,829 Tyler Nov. 27, 1945 

